1. Field of the Invention
The present invention relates to an optical pickup lens used in an optical system for performing recording or playback of an optical disc.
2. Description of Related Art
The recording capacity of an optical disc is ever increasing, and the recording density per unit area is also increasing. When reading information on an optical disc, light from a light source of an optical disc apparatus, after passing through an optical path including transparent components such as a wave plate and a collimator lens, is focused by an optical pickup lens to form an optical spot on an optical disc, thereby reading the information on the optical disc. Normally, light that is emitted from a laser light source enters an optical pickup lens after it is converted into parallel light by a collimator lens or the like. For an optical pickup lens which is used for reading a high-capacity optical disc, laser light with a wavelength of 410 nm or shorter is used, and a numerical aperture NA is generally set to 0.84 or larger.
An example of an optical pickup lens of a related art is disclosed in Japanese Unexamined Patent Application Publication No. 2002-156579. The objective lens for an optical disc which is disclosed therein is a double-sided aspherical single lens with a numerical aperture NA of 0.7 or larger, in which the center thickness of the lens is longer than a focal length. Another example is disclosed in Japanese Unexamined Patent Application Publication No. 2001-324673. The objective lens which is disclosed therein has an aspherical surface on one side and satisfies 1.1≦d1/f≦3 where d1 is an on-axis lens thickness and f is a focal length. Yet another example is disclosed in Japanese Unexamined Patent Application Publication No. 2002-303787. The objective lens which is disclosed therein is a double-sided aspherical single objective lens with a numerical aperture NA of 0.75 or larger which satisfies 1.75<n and 35<v where n is a refractive index for at least one use wavelength and v is Abbe number on a line d.
It is necessary to maintain good off-axis characteristics in order to mount an optical pickup lens to a pickup. However, if a numerical aperture NA exceeds 0.80, it is difficult to maintain good characteristics for both on-axis aberration such as spherical aberration and off-axis aberration such as astigmatic aberration and comatic aberration of a pickup lens. Particularly, a biconvex lens in which a surface on the side close to a laser light source (surface R1) and a surface on the opposite side (surface R2) are both convex is difficult to have good angle of view characteristics.
Further, if a numerical aperture NA exceeds 0.80, a working distance (WD) which indicates a distance between an optical pickup lens and an optical disc becomes smaller. The decrease in the working distance is particularly significant in a meniscus lens in which the surface R2 is not convex, thus increasing the risk of collision between an optical disc and an optical pickup lens.